Warehouses, distribution centers and fulfillment centers typically receive full pallets of uniform products. In many cases, the products are depalettized and stored individually in a storage area. When specific orders need to be prepared, the products are transported to a palletizing cells, which can be either manual, automated or robotized.
Because of the wide variety of products, a special and versatile end of arm tool is required when the robotized approach is taken.
Typical tools to grip products are equipped with vacuum cups or pads. Such tools are economical and easy to use. While they can be used with closed boxes or cartons, they show limitations to grip trays, stretch wrapped products or the like. Problems may also occur if the top flaps of the boxes are not glued or tapped adequately. Because of the multiplication of product sizes and formats found in warehouses and distribution centers, a more robust and versatile tool is required.
The use of a fork-type tool allows solving the above-mentioned drawback. Traditionally, the bottom forks of such tools can be extended or retracted. Therefore, when a product is gripped, the bottom forks are extended to go underneath the product, while they are retracted when the product is dropped in place. This type of tool is popular, considering that it can handle virtually any type of product in a reliable fashion.
However, a first drawback of typical fork-type gripping tool relates to the width of the bottom forks. Ideally, it should be wide enough to be able to pick and handle products of various dimensions, but when it is too large, it limits the capacity to drop narrow products on the pallet considering that the wide forks might interfere with products already in place on the pallet. A second drawback is the cycle time required for a conventional fork tool to pick and place products.
U.S. Pat. No. 8,235,436 B2, issued on Aug. 7, 2012, to Ryf and being titled “Bundle Gripper for a Palletizing Machine and Method for the Palletizing of Bundles” describes a bundle gripper for a palletizing machine handling bundles, wherein a bundle is fed from a feeding device to the bundle gripper, the bundle resting against an end stop. Ryf tool has at least two gripper units, and gripper fingers that are extendable. A drawback of the tool by Ryf is that providing movable fingers renders the tool heavier and more complex, therefore requiring additional maintenance.
U.S. Pat. No. 6,579,053 B1, issued on Jun. 17, 2003 to Grams et al. and being titled “Robotic Containerization and Palletizing System” describes a robotic system using an end effector comprising a plurality of rotatable fingers designed specifically to handle trays, pallets or tubs.
The approach taken by Grams et al is dedicated to handle objects with geometry similar to trays and the like, and is therefore not suitable for palletizing mixed load products in a warehouse.
U.S. Pat. No. 8,807,912 B2, issued on Aug. 19, 2014 to Liebheit and being titled “Infeed Station and Stack Gripper of a Palletizing System and Method for Transferring Stacks from an Infeed Station to a Stack Gripper” describes a tool and method for transferring stacks.
The approach taken by Liebheit is specific to stack handling where a stack conveyor and at least two stack chambers are used. Liebheit uses the gripper described in details in U.S. Pat. No. 8,235,436 B2 discussed above.
United States Publication No 2010/0218464 A1, published on Sep. 2, 2010 to Baumann and being titled “Gripper for an Automated Manipulator and Method for Operation of the Gripper” describes a tool including bottom forks to secure the product from underneath, and a stop and a counter-stop to firmly hold the product on two opposite sides. This tool is designed to firmly hold a variety of products, but is not suitable for palletizing at high rates because of its multi-step sequence. Once the tool is positioned close to the product to be picked, a first motor moves the bottom forks underneath the product. The counter-stop is then lowered and finally the stop is moved forward to positively hold the product on two opposite sides. The reverse sequence is performed when placing the product on the pallet. It results that the manipulator is in a waiting position for too long and is not suitable for applications where high throughput is required.
Baumann's gripper illustrates a problem common to most known grippers, which are designed in such a way that the robot has to completely stop to enable the bottom forks to go underneath the product and then to lower the top pad to securely hold the product before the robot can start its retrieving movement to bring the picked product to the pallet.
Many systems from the prior art include an upper mobile pad or plate to secure the product from the top. One known approach is to use a servo-driven axis to lower the upper pad to the desired height. This approach is fast to modify and adapt the tool's opening from one product to the other, but does not precisely adapt to the real product's dimension because the exact dimensions of the cases are rarely their nominal values. If the real dimension is slightly higher that the nominal value, the upper pad can crush the product and possibly damage at least the packing if not the product itself. Similarly, if the product real height is less that its nominal value, the upper pad will not hold the product properly. Another approach from the prior art is to use pneumatic driven axes to move a plate to pick the product. This approach is slower compared to the one previously mentioned because the upper pad returns to the uppermost position between each product. When a small product is picked, the long downward movement increases the picking time, thus the overall cycle time. On the other hand, this approach allows applying a predetermined pressure that enables the tool to firmly hold the product without damaging the packing or the product regardless of its real dimensions.